In recent years, inkjet recording apparatuses that form images using an inkjet recording system have been widely used as image forming apparatuses due to obtaining high-definition image quality comparable to silver-halide prints having become possible from rapid advancements in recording technology. For such inkjet recording apparatuses, increased speed in image formation has been demanded. However, in a case of performing image formation at high speed using an inkjet recording apparatus, a recording medium carrying the ink passes between a pair of ejection rollers and is ejected before the ink penetrates the recording medium such as paper. On this occasion, the ink may adhere to the ejection rollers (offsets). In this case, image defects caused by offset come to easily occur in the formed image. In order to suppress the occurrence of offset, it has been considered to decrease the amount of ink ejected. However, if decreasing the amount of ink ejected, it may not be possible to obtain an image of a desired image density.
In view of the above-mentioned situation, as an ink for inkjet recording apparatuses that excels in penetrability to the recording medium and can form a favorable image, a water-based ink has been proposed that is characterized in containing a wetting agent and a pigment dispersion made by dispersing pigment as a coloring agent in water using a resin, and further containing 2-ethyl-1,3-hexanediol.
In the above ink, 2-ethyl-1,3-hexanediol is used as a component for improving the penetrability of the ink into the recording medium. However, with this ink, in a case of the content of 2-ethyl-1,3-hexanediol being small, because the penetrability of the ink into the recording medium cannot be sufficiently raised, and contamination of the recording medium caused by offset may occur. In addition, with this ink, in a case of the content of 2-ethyl-1,3-hexanediol being great, since the penetrability of the ink into the recording medium will be too high, the coloring agent will penetrate inside of the recording medium along with liquid component contained in the ink. There is a problem in that there is a possibility for the image density of the formed image to be less than a desired value if the coloring agent penetrates to inside of the recording medium.
Furthermore, the above ink has a problem in that, depending on the type of wetting agent, in the case of having been left for a long time without the recording head filled with ink being capped, the wetting agent solidifies in the recording head and considerable thickening of the ink occurs, and it is difficult to recover the ejection state of ink droplets to the state prior to being left, even if performing a recovery operation of carrying out purging and wiping on the recording head.
In addition, in a case of using inkjet recording apparatus of a line head-type recording system, which is able to form images at high speed, it is necessary to increase the amount of ink ejected in order to form an image having such a desired image density, since it is not possible to perform overlapped drawing of the image. Furthermore, with the inkjet recording apparatuses of a line head-type recording system using inks of several colors, ink droplets are intermittently ejected from a plurality of heads onto the recording medium moving in the recording apparatus. In such a case, drying of the ink droplets landed on the recording medium from the head progresses in a period until ink droplets are landed from a next head; therefore, penetration of the ink droplets landed from the next head into the recording medium tends to be hindered.
For this reason, in a case of using inkjet recording apparatus of a line head-type recording system, the problem of offset occurrence in particular tends to arise. Therefore, with above-mentioned ink for inkjet recording apparatuses, it is very difficult to balance the suppression of the occurrence of offset and the forming of images having a desired image density in an inkjet recording apparatus of a line head-type recording system.